Plasma accelerator arrangement

ABSTRACT

For a plasma accelerator arrangement in particular for use as an ion thruster in a spacecraft, a structure is proposed in connection with which an accelerated electron beam is admitted into an ionization chamber with fuel gas, and is guided through the ionization chamber in the form of a focused beam against an electric deceleration field, said electric deceleration field acting at the same time as an acceleration field for the fuel ions produced by ionization. The arrangement generates a focused beam of a largely neutral plasma with a high degree of efficiency. Configurations for electric and magnetic fields for guiding and focusing the beams are given by way of example.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of GERMAN Application No.198 28 704.6 filed on Jun. 26, 1998. Applicants also claim priorityunder 35 U.S.C. §120 of PCT/DE99/01708 filed on Jun. 11, 1999. Theinternational application under PCT article 21 (2) was not published inEnglish.

DESCRIPTION

The invention relates to a plasma accelerator arrangement. Plasmaaccelerators (ion thrusters, electric propulsion systems—EPS) are veryimportant as thrusters in spacecraft both for satellites orbiting closeto the earth and geo-stationary satellites, and for space missionsoutside of the earth orbit. The ratio of driving pulse to mass of thefuel, which is used as a measure of the degree of efficiency of thedrive system is substantially more favorable for plasma acceleratorsthan for conventional chemical drive system, with the result that theproportional weight of the fuel is reduced. Such a reduction is ofparticular importance for space applications. A noble gas with a highatomic weight, in particular xenon is frequently employed as the fuel.

In conjunction with grid ion thrusters, for example U.S. Pat. No.4,838,021, a plasma is produced in an ionization chamber from neutralgas by high frequency or electron bombardment. The positively chargedions are accelerated in an applied electrical field in the direction ofejection toward a grid electrode. For the electrical neutralization, acurrent of free electrons has to be added to the accelerated ioncurrent. The neutralized current of plasma exits from the drive systemat high speed and accelerates the spacecraft in the opposite direction.Owing to the space charging effects, the density of the ion current islimited, and the drive systems of this type require large cross sectionscombined with only moderate reaction propulsion effects.

In conjunction with drive systems according to the Hall principle, forexample EP 541,309 A1, a ring-shaped ionization chamber is penetrated byan electrical acceleration field parallel with the axis of the ring, anda radial magnetic field. From an external electron source, electrons areguided into the ionization chamber containing neutral xenon gas againstthe direction of ejection of the ions. In said ionization chamber, theelectrons are forcibly guided into spiral orbits because of the magneticfield, and the running distance within the ionization chamber ismultiplied in this way versus the direct distance to the anode, withionizing interaction with the fuel gas being increased in this way aswell. Secondary electrodes are affected by the magnetic deflection aswell and are accelerated in the electrical field. Furthermore, the givenfield configuration largely prevents the development of space chargingzones that might cause screening of the electrical acceleration fieldfor the positive fuel ions. The acceleration of the positive ionstherefore takes place in a largely neutral plasma. Such an arrangementpermits distinctly higher current densities than a grid ion drive systemarrangement; however, it exhibits an only moderate degree of efficiencydue to large widening of the angle of the ejected ion current.

DE 1,222,589 B describes a device for generating a spacecharge-neutralized beam of charged particles in connection with which abeam of highly accelerated electrons is admitted into an ionizationchamber along its longitudinal axis and guided by a magnetic fieldextending parallel with the longitudinal axis. An arc discharge in theionization chamber generates from admitted gas slow electrons andpositively charged ions. While the latter are accelerated in thedirection of the primary electron beam by an ion acceleration electrodeand exit from the ionization chamber together with decelerated electronsof the primary electron beam in the form of a neutral plasma beam, theslow electrons of the gas discharge oscillate between the electrodes onthe inlet and outside sides, guided by the magnetic field extendingparallel with the longitudinal axis. The accelerated ions and thedecelerated electrons of the electron beam exit from the arrangement asa neutral plasma beam.

The present invention is based on the problem of proposing a plasmaaccelerator arrangement in particular in the form of an ion thruster inspacecraft, with an enhanced degree of efficiency.

The invention is described in patent claim 1. The dependent claimscontain advantageous embodiments and further developments of theinvention.

In conjunction with the arrangement as defined by the invention, thefocused electron beam introduced into the ionization chamber firstinitiates ionization of the neutral fuel gas present in or admitted intosaid ionization chamber. The secondary electrons released in the courseof ionization are accelerated in the opposite direction in theelectrical field provided for accelerating the positive ions andthemselves act again in an ionizing manner. Following initiation of theionization process by the electron beam, the secondary electrons mayassume the main part of the further ionization.

A further important effect of the admitted electron beam is thatfocusing of a beam of ions accelerated in the electric accelerationfield is favored by compensating its positive space charge with theelectron beam, so that no screening of the accelerating electric fieldtakes place. The acceleration field for the positive ions has adecelerating effect on the electrons of the electron beam running in thesame direction as the accelerated ion current, so that the spacecharging density of the electron beam increases in the direction of thelongitudinal axis of the ionization chamber, which advantageouslycorresponds with the concentration of the ion beam desired in the endsection of the ionization chamber. The average speed of the electrons ofthe electron beam and the potential gradient of the acceleration fieldfor the ions, which corresponds with a potential increase for theelectrons, are preferably coordinated with each other in such a way thatat the end of the acceleration path for the ions (or decelerating pathfor the electrons of the electron beam), the average speeds of theelectrons of the electron beam and of the ions of the accelerated ioncurrent are approximately the same, so that an approximately neutralplasma exits at the end of the acceleration path. The average speedsdiffer preferably by less than the factor 10.

The electron beam acts through its negative space charge over the entirelength of the ionization chamber also as a central means for attractingthe positive ions and supports focusing of the accelerated ions in afocused electron current and at the same time compensates mutualrepelling of the ions. A widening of the electron beam can becounteracted by a beam guiding and/or beam focusing system consisting ofmagnetic and/or electric fields. Advantageous is especially a magneticbeam guiding system with a field extending within the zone of the beamsubstantially parallel with the direction of the beam and in relation tothe longitudinal axis of the ionization chamber. Electrons of theelectron beam with a component of the motion acting perpendicular to thelongitudinal axis are forced by the magnetic field into a spiral orbitaround the axis of the beam. Magnetic beam guiding systems are known perse from electron-beam tubes in a great variety of forms and inparticular in conjunction with travelling-wave valves in the form ofpermanently periodic magnet arrangements with field direction reversalsoccurring along the central axis, where the field has strong radialcomponents as well. Reference is made to such known beam-guiding systemsfor the purpose of disclosure. Traveling-wave valves with such magnetarrangements are known, for example from DE 2,652,020 B2 orDE-AS-1,491,516.

A magnetic field system is also advantageously suitable for forcing theslow secondary electrons from the ionization processes that areaccelerated in the electrical acceleration field for the positive ionsin the opposite direction, into spiral-shaped or similarly curvedorbits. In this way, the electrons are prevented from rapidly impactingan electrode following against the longitudinal axis of the ionizationchamber, on the one hand, and the probability that a secondary electrontriggers one or several further ionization processes is distinctlyincreased, so that the fuel gas can be primarily ionized by thesecondary electrons. On the other hand, the positive space charging ofthe slow ions caused in the ionization process by the longer dwellingtime of the secondary electrons is partially compensated. Finally, theelectrons also can be largely kept within the respective potential stageby the magnetic field and finally can be guided to the electrode that isnext against the longitudinal direction, so that higher losses of energycaused by secondary electrons that are accelerated over longer distancescan be avoided. Reversing of the secondary electrons into curved orbitsabout the direction of the field acting on the electrons in anaccelerating manner is particularly effective if the field directions ofthe electric and the magnetic fields are extending vertically one on topof the other. The electric fields and the magnetic fields are thereforeadvantageously realized in such a way that the field lines cross eachother in the predominating part of the ionization chamber, in particularin more than 90% of its volume. The angle enclosed by the electric fielddirection and the magnetic field direction preferably amounts to between45° and 135° in at least 50% of the volume of the ionization chamber.Both the magnetic and the electric field show in this connection in adistinct to predominant manner field components extending parallel withthe longitudinal axis, and the mean field directions of the electricfield and the magnetic field are preferably coinciding on thelongitudinal axis of the ionization chamber. In connection with amagnetic field with field direction changes along the longitudinal axis,the mean field direction is to be understood without taking into accountthe polarity.

An advantageous arrangement for the present purposes provides for afield configuration in which electrodes for generating the electricfield and poles of the magnetic field successively follow each other inthe direction of the longitudinal axis in an alternating manner, and inwhich preferably electrodes and/or pole shoes are arranged on the sidewall of the ionization chamber. The fields are preferablyrotation-symmetric with respect to the longitudinal axis and showmaximum and minimum values of their field strength on the longitudinalaxis. In the simplest single-stage structure, two electrodes arearranged spaced from each other in the longitudinal direction of theionization chamber, and three pole shoes surrounding the ionizationchamber are also arranged spaced from each other in the longitudinaldirection, and arranged with changing polarity in such a way that one ofthe two electrodes is enclosed in each case between two pole shoes. Inthe longitudinal direction, the electrodes each are at leastapproximately disposed near maximums of the magnetic field strength onthe longitudinal axis, and the minimum of the magnetic field strength onthe longitudinal axis in the site of the field direction reversalcoincides at least approximately with the maximum of the electric fieldin the direction of the longitudinal axis.

Particularly advantageous is a multi-stage arrangement in which themagnetic field has a plurality of field direction reversals on thelongitudinal axis and the pole shoes surrounding the ionization chamberin the form of a ring successively follow each other with alternatingpolarity in the longitudinal direction and each are inserted between twoelectrodes of the electric electron arrangement. The plurality ofelectrodes form potential stages. However, as opposed to the magneticfield, the electrical field shows no field direction reversal on thelongitudinal axis. The electrical potential changes monotonously fromstage to stage in the longitudinal direction of the ionization chamber.Outside of the longitudinal axis, the fields of the two types of fieldextend crossed in relation to each other, whereby the angle of between45° and 135° enclosed by the field directions crossing each other ispreferably disposed in at least 60% of the volume.

The electric and the magnetic fields can be advantageously coordinatedwith one another in a way such that a secondary electron produced byionization within the zone of an electric potential stage between twoelectrodes directly neighboring on each other is kept by the magneticfield with said stage, if possible, and possibly guided to the electrodethat is located next against the longitudinal direction upon effectingone or several further ionization processes.

While the electrons are subjected to strong influence of the magneticfield because of their low mass, the movement of the ions issubstantially determined only by the electric fields. The ions areaccelerated in the direction of the potential gradient and concentratedtoward the longitudinal axis, whereby such focusing is decisivelyjointly effected also by the field lines occurring between adjacentelectrodes. The ions are therefore capable of absorbing on the averagefrom the electric field energy from a plurality of potential stages,whereas the energy losses caused by the secondary electrons trapped bythe electrodes remain low, on the other hand, because their movement islimited to one or two potential stages, so that the result is a highdegree of efficiency in the conversion of electric energy intomechanical energy.

The preferably ring-shaped electrodes, in particular the intermediateelectrodes of a multi-stage arrangement enclosed between two furtherelectrodes, have a flat expanse advantageously in the longitudinaldirection for reliably catching secondary electrons, whereby the lengthof the intermediate electrodes in the direction of the longitudinal axispreferably amounts to at least 30%, in particular to at least 80% of thespacing from the next electrode in each case.

For obtaining the described field properties with limitation of themovement of the secondary electrons and focusing of the positive ions byfield lenses, the diameter of an intermediate electrode preferablyamounts to less than 300%, in particular less than 100% of the electrodelength in the direction of the longitudinal axis.

The generation of a focused electron beam in the form of a central beamor a hollow beam is known in many variations from the technology of thecathode-ray tube, so that details in this regard are omitted here andreference is made to arrangements known from the prior art for exampleDE 1,222,589 B or DE 2,931,746 C2. For the present invention, anelectron current detached from a cathode is focused to form a laminarbeam, for example by means of an electron optical system of the Piercetype, and admitted into the ionization chamber along the longitudinalaxis. The inlet zone of the electron beam, where the latter enters theionization chambers, is advantageously realized in the form of a barrierfor ions generated in the ionization chamber in order to prevent ionsfrom being guided by penetration of the cathode potential to the cathodeand causing losses or a degradation of the electron emission power ofthe cathode, or in order to keep such an undesirable ion current atleast low. The first electrode, for example, can be realized as an ionbarrier in the form of an annular shutter having a small diameter of theperforated shutter vis-a-vis the diameter of the ionization chamber.Within the ionization chamber, the beam is guided by the describedmagnetic field in the form of a focused beam.

The primary electrons of the focused electron beam are decelerated withthe potential difference developed for the acceleration of the ionswithin the acceleration path between the first and the last electrodesof the preferably multi-stage electrode arrangement, which is preferablysubstantially identical with the ionization chamber. The electrons reachthe end of the deceleration path without additional loss of energy witha residual speed that is to be much lower than the speed at the start ofthe deceleration path (deceleration path for the electrons). The voltageof the last electrode is for that purpose advantageously slightlypositive versus the cathode. Without fuel gas in the ionization chamber,the electrons can be trapped at the end of the acceleration path withlow residual energy by the last electrode then acting as the collector.

In the real case, with fuel gas in the ionization chamber, some of theprimary electrons give off energy on their way through the ionizationchamber due to interaction with the fuel gas, in particular due toexcitation and ionization of fuel gas, and loose speed. The loss ofspeed based thereon, however, is very minor as compared to the initialspeed and can be taken into account and particularly determinedempirically when the potential of the last electrode is adjusted forobtaining a neutral plasma in the exiting beam of plasma.

The feed of the fuel gas preferably takes place through lateral wallopenings of the ionization chamber in an zone that is spaced both fromthe inlet side of the electron beam and the outlet side of the plasmabeam. The feed zone is preferably removed from the inlet of the electronbeam with a spacing amounting to between 10% and 40% of the length ofthe ionization chamber in the longitudinal direction.

The features specified above and in the claims can be advantageouslyrealized both individually and in combinations.

The invention is explained in detail in the following with the help ofexemplified embodiments and by reference to the drawings, in which:

FIG. 1 is a longitudinal section through a multi-stage structure.

FIG. 2 shows the field curves for a multi-stage arrangement; and

FIG. 3 shows the field curves for a single-stage arrangement.

The arrangement sketched in FIG. 1 as a longitudinal section along alongitudinal axis “Z” has an ionization chamber “IK” around thelongitudinal axis “Z”. Said ionization chamber is structured, forexample rotation-symmetrically around the longitudinal axis “Z”. Thelongitudinal expanse of the ionization chamber “IK” in the direction ofthe longitudinal axis “Z” is assumed to be substantially greater thanthe diameter “DK” of the ionization chamber perpendicular to thelongitudinal axis “Z”. The ionization chamber “IK” is assumed to bedefined transversely in relation to the longitudinal axis by acylindrical side wall.

The magnetic pole shoes “PP” and the electrodes E1, E2 to E5 arearranged along the side wall, whereby the pole shoes and the electrodesare realized in a rotation-symmetrical manner at least on their sidespointing at the longitudinal axis “Z”. The magnetic pole shoes “PP”deflect the magnetic flux generated by the permanent magnet “PM”, thelatter being arranged radially spaced from the ionization chamber “IK”in the radial direction, so that each pole shoe “PP” is forming amagnetic pole, whereby the pole shoes successively arranged one directlyafter the other in the longitudinal direction, are forming magneticpoles with the opposite polarity. The magnetic field developing in theionization chamber “IK” between the pole shoes, therefore, shows areversal of the field direction in the location of each pole shoe “PP”.Such an arrangement of the magnetic field is sufficiently known per sefrom the technology of the travelling-wave valves as a permanentlyperiodic magnetic system.

The electrodes E1 to E5 located between the pole shoes “PP” are actedupon by the different potentials A1, A2 to A5. The electrode arrangementis supplemented by a cathode “K” and an anode “E0”. The cathode “K” andthe anode “E0” form a beam generation system for generating a focusedelectron beam “EB” from a laminar electrode current “ES”. Beam systemsfor generating and focusing an electron beam are known in a great numberof variations from the prior art as well. The anode electrode E0 and theelectrodes E1 to En defining the ionization chamber laterally jointlyform an electrode arrangement with a potential gradient that ismonotonous for ions, ranging from the potential E0 up to the potentialA5 of the electrode E5, with A0>A1>A2>A3>A4>A5 for positively chargedions. For the negatively charged ions of the electron beam “EB”, thepotential series A0 to A5 forms a braking potential which is constantlyreducing the starting speed of the electrons of the electron beam “EB”as it progresses along the longitudinal axis “Z”. The voltage of cathodeK is selected slightly negative as opposed to the last electrode E5, sothat the electrons of the electron beam “EB” still have a low residualspeed after passing through the ionization chamber up to the electrodeE5. The electron beam “EB” is guided in the ionization chamber as afocused beam by the magnetic field “HK” built up between the pole shoes.

The neutral fuel gas “TG” is fed into the ionization chamber through theside wall. The electrons of the electron beam “EB” interact with theneutral fuel gas and effect a partial ionization of the gas. Thepositively charged ions generated by such interaction are accelerated inthe direction of the potential gradient from A0 to A5 and are focused ontheir way toward the longitudinal axis “Z” by the focused electron beam“EB” and by the field lenses formed by the successively arrangedelectrodes. The secondary ions liberated in the ionization process havea very low speed at the start in the statistically varying direction.The secondary electrons are accelerated in the electrical field “EK”between the individual electrodes, whereby the direction of accelerationis opposing the direction of acceleration of the ions. The acceleratedsecondary electrons are reversed by the magnetic field “HK” that ispresent in the ionization chamber “IKL” at the same time, and are forcedinto curved orbits around the direction of the accelerating electricalfield. The dwelling duration of the electrons in a stage between twoelectrodes is substantially prolonged in this way and the probabilitythat such a secondary electron might trigger further ionization processis highly increased. The secondary electrons are finally trapped by anelectrode following in the direction of the anode “E0”. The longerdwelling time of the electrons in the ionization chamber between twoelectrodes until impacting an electrode also contributes to the factthat rapid build-up of a positive space charge by the positively chargedions and thus screening of the field accelerating the ions are avoided.

The ionization of the fuel gas “TG” by both the primary electrons of theelectron beam EB and the secondary electrons from preceding ionizationprocesses substantially distributes itself over the entire length of theionization chamber. The ions accelerated along the potential gradientbetween A0 and A5 in the direction of the longitudinal axis “Z”, andfocused around the longitudinal axis “Z”, together with the deceleratedelectrons of the focused electron beam “EB”, jointly form at the outlet“KA” of the ionization chamber a largely neutral plasma beam “PB”withonly minor beam divergence.

The anode electrode E0 is realized at the same time as an electronbarrier and has the form of a shutter electrode with a small diameter ofthe shutter opening as compared to the diameter “DK” of the ionizationchamber. In FIG. 1, the voltage of the electrodes E0 to E4 are plottedbased on the last electrode E5 as exemplary values for the potentialstages. The voltage of the cathode K is slightly negative versus thelast electrode E5. The poles of the magnetic arrangement aredistinguished by the designations “S” and “N” in the usual manner.

FIG. 2 qualitatively shows the field curve and the distribution of theelectron charge along the longitudinal axis “Z” for a cutout from thestructure according to FIG. 1. In the representation of FIG. 2, also theelectrodes E0, E1 etc., as well as the pole shoes “PP” are indicated intheir positions along the Z-axis in addition to the field curves and thecharge distributions of the electrons. The primary electron beam “EB” issymmetrically plotted on both sides of the longitudinal axis “Z”,whereas the accumulations “EC” of the secondary electrons are plottedonly on one side of the longitudinal axis “Z” for the sake of betterclarity. The field strengths are plotted on the longitudinal axis orwithin its immediate proximity with “Ez” and “Hz”. The longitudinal axis“Z” forms the abscissa of the representation and the ordinate indicatesthe field intensities “Ez” and “Hz” qualitatively. The electric field“Ez” on the longitudinal axis is minimal in the positions of theelectrodes E0, E1 etc., and maximal in the center between theelectrodes. No reversal takes place in this connection in direction, sothat the electric field strength does not change the sign in the presentrepresentation. The magnetic field strength in the Z-direction showsminimums in the locations of the pole shoes “PP”, and maximums betweentwo adjacent pole shoes. As opposed to the electric field, a reversal ofthe field direction occurs for the magnetic field on each of theindividual pole shoes, which, in the sketch, represents a passagethrough the longitudinal axis “Z” imagined as the zero line, and whichcan be treated as a change in sign. The secondary ions generated inionization processes are accelerated by the electric field in theionization chamber and forced into curved orbits by the magnetic field.An accumulation of electrons, i.e. an increased concentration ofelectrons occurs in the ring-shaped zones “EC” around the longitudinalaxis “Z”; said zones are approximately located at the minimums of theelectric or maximums of the magnetic field with respect to thelongitudinal direction.

FIG. 3 shows in a comparative representation as in FIG. 2 field curvesand electron distributions for a single-stage arrangement with the twoelectrodes E1, E2 acted upon by the potentials A0 and, respectively, A1,as well as with a magnetic arrangement with the three pole shoes PP1,PP2 and PP3, of which two pole shoes enclose in each case one of the twoelectrodes E1 and E2, respectively. The field arrangement of such asingle stage shows, in a way similar to the distribution sketched inFIG. 2, minimums of the electric field strength on the axis in thelocations of the electrodes E1, E2, as well as a maximum of the electricfield strength “Ez” on the axis between the two electrodes. The magneticfield strength “Hz” on the longitudinal axis “Z” reaches a minimum inthe zone of the center pole shoe PP2, where a reversal of the fielddirection takes place as well. Maximums of the magnetic field strengthare located again in the zone of the electrodes E1 and E2, where theelectric field strength “Ez” has minimums on the axis. The mode ofoperation in the occurrence of the ring-shaped electron clouds“EC”around the longitudinal axis “Z” corresponds with what has beenstated above with respect to FIG. 2.

The invention is not limited to the exemplified embodiments describedabove, but can be modified in a number of ways within the scope of theskills of the expert. Especially in regard to the dimensions of theionization chamber and the ratios between the electrodes, the electrodespacing and the diameters of the electrodes in terms of size, amultitude of variations are conceivable that are adapted to theindividual case. The spacings of the electrodes and/or of the poleshoes, as well as the length of the electrodes in the direction of thelongitudinal axis are not necessarily constant for all stages inmulti-stage arrangements. The potential gradient between the first andthe last electrodes is not necessarily linear, but may assume also anonlinear course in the individual case. The plasma acceleratorarrangement is not limited to the described preferred case ofapplication for an ion thruster for a spacecraft, but rather can beadvantageously employed also for contactless metal working operationswith the application of high power densities in operations such as, forexample welding, soldering, cutting or the like, including also theworking of high-melting metals.

What is claimed is:
 1. A plasma accelerator arrangement, for generating a plasma beam, comprising: a) an ionization chamber disposed around a longitudinal axis; b) an electrode arrangement disposed in said ionization chamber and comprising a plurality of electrodes having different electric potentials for generating an electric potential gradient and an electrostatic field for the acceleration of positively charged ions wherein the electrostatic field extends along said longitudinal axis which defines an acceleration path; c) an electron beam emitter coupled to said ionization chamber spaced apart from said plurality of electrodes in said electrode arrangement said electron beam emitter for emitting an electron beam along said longitudinal axis into said ionization chamber; and d) a magnetic beam guiding system which creates a magnetic field for guiding the electron beam along said longitudinal axis wherein said magnetic beam guiding system has at least one direction reversal of said magnetic field along said longitudinal axis and said acceleration path.
 2. The arrangement as in claim 1, wherein said magnetic beam guiding system contains at least one permanent magnet.
 3. The arrangement as in claim 1, wherein said plurality of electrodes in said electrode arrangement comprises at least one intermediate electrode.
 4. The arrangement as in claim 3, wherein said ionization chamber has at least one lateral wall and said at least one intermediate electrode is disposed on said at least one lateral wall.
 5. The arrangement as in claim 3, wherein each of said at least one intermediate electrode has a length that is at least 30% and at most 80% of a spacing from an adjacent electrode of said plurality of electrodes.
 6. The arrangement as in claim 3, wherein said at least one intermediate electrode has a diameter that is less than 100% of its length along said longitudinal axis.
 7. The arrangement as in claim 1, wherein said magnetic beam guiding system has a plurality of magnetic poles which are arranged in an alternating manner with each of said electrodes in said electrode arrangement being spaced apart in the direction of said longitudinal axis, wherein said plurality of magnetic poles arranged along said longitudinal axis create a plurality of direction reversals along said acceleration path.
 8. The arrangement as in claim 1, wherein said electrostatic field and said magnetic field extend over more than 90% of the volume of the ionization chamber.
 9. The arrangement as in claim 8, wherein said ionization chamber is designed so that across more than 60% of a volume of said ionization chamber, said electrostatic field direction and said magnetic field direction is between 45° and 135°.
 10. The arrangement as in claim 1, wherein said plasma beam comprises a plurality of electrons having a mean speed approximately equal to a mean speed of said plurality of positively charged ions.
 11. The arrangement as in claim 10, wherein said mean speed of said plurality of electrons differs from said mean speed of said plurality of positively charged ions by a factor of no more than
 10. 12. The arrangement as in claim 1, further comprising an ion barrier disposed inside of said ionization chamber wherein said acceleration path for ions is sealed toward a side of entry of the electron beam by said ion barrier.
 13. The arrangement as in claim 1, wherein at least one electrode of said plurality of electrodes is located at a start of said acceleration path for ions wherein said electrode is a shutter electrode having a central opening for the electron beam wherein said opening has a diameter that is substantially smaller than a diameter of the ionization chamber.
 14. The arrangement as in claim 1, further comprising a neutral gaseous fuel feed for feeding fuel laterally into said ionization chamber.
 15. The arrangement as in claim 14, wherein said fuel feed is disposed in a zone between 10% and 40% of a distance along a length of said chamber from a side entry of said electron beam.
 16. The arrangement as in claim 1, wherein said ionization chamber has a length that is at least three times greater than its diameter.
 17. The arrangement as in claim 1, wherein said acceleration path and said ionization chamber extend coaxially.
 18. The arrangement as in claim 1, further comprising a cathode, disposed in said ionization chamber for generating a focused electron beam, and wherein said cathode has a voltage that is negative compared to a voltage of at least one of said plurality of electrodes, which is disposed at an outlet of said ionization chamber.
 19. A plasma accelerator arrangement, for generating a plasma beam, comprising: a) an ionization chamber disposed around a longitudinal axis and having an outlet opening formed coaxially with said longitudinal axis and an acceleration path in said ionization chamber, said outlet opening allowing a plasma beam to leave said ionization chamber; b) an electrode arrangement disposed in said ionization chamber and comprising a plurality of electrodes having different electric potentials for generating an electric potential gradient and an electrostatic field for the acceleration of positively charged ions wherein the electrostatic field extends along said longitudinal axis which defines said acceleration path; c) an electron beam emitter coupled to said ionization chamber spaced apart from said plurality of electrodes in said electrode arrangement said electron beam emitter for emitting an electron beam along said longitudinal axis into said ionization chamber; and d) a magnetic beam guiding system which creates a magnetic field for guiding the electron bean along said longitudinal axis wherein said magnetic beam guiding system has at least one direction reversal of a magnetic field along said longitudinal axis and said acceleration path.
 20. A plasma accelerator arrangement, for generating a plasma beam, the plasma accelerator in the form of an ion thruster or electric propulsion system comprising: a) an ionization chamber disposed around a longitudinal axis and having an outlet opening formed coaxially with said longitudinal axis and an acceleration path in said ionization chamber, said outlet opening allowing a plasma beam to leave said ionization chamber; b) an electrode arrangement disposed in said ionization chamber and comprising a plurality of electrodes having different electric potentials for generating an electric potential gradient and an electrostatic field for the acceleration of positively charged ions wherein the electrostatic field extends along said longitudinal axis which defines said acceleration path; c) an electron beam emitter coupled to said ionization chamber spaced apart from said plurality of electrodes in said electrode arrangement said electron beam emitter for emitting an electron beam along said longitudinal axis into said ionization chamber; and d) a magnetic beam guiding system which creates a magnetic field for guiding the electron beam along said longitudinal axis wherein said magnetic beam guiding system has at least one direction reversal of a magnetic field along said longitudinal axis and said acceleration path. 